To refine pig iron into steel in converters, blowing with pure oxygen is generally used, which can be carried out in two different ways:
either, from top to bottom by means of a water-cooled lance introduced through the nose of the converter,
or, from bottom to top, vertically or obliquely, by means of tuyeres with two or several concentric tubes, generally disposed in the refractory bottom of the converter, and protected against too rapid wear by a peripheral protective agent introduced into the outer tube of each tuyere.
Blowing with pure oxygen by lance, from top to bottom, in vertical converters presents the following two disadvantages, among others:
(a) production of abundant red iron oxide fumes throughout conversion, which constitutes a significant loss of iron;
(b) stirring of the metal bath due exclusively to localized decarbonization in the reaction region whose intensity can only be affected by biasing the decarbonization speed, which is a function of the flow-rate of pure oxygen, which also has to comply with a certain number of constraints. This stirring of the bath by the single decarbonization limits the possibility of action being taken during the conversion process. In particular, a physico-chemical imbalance which is sometimes considerable can occur, at certain times in the operation, between the iron oxide content of the slag and the carbon content of the metal bath, with some awkward consequences which are well known to steelmakers. Similarly, the loss of stirring at the end of refining, at low carbon contents, means that the end of the operation is not completely controlled and consequently prevents the steel from being accurately made in the converter with all the required characteristics. Additional metallurgical measures then have to be taken downstream of the converter to remedy this.
With this type of blowing with oxygen by lance, from top to bottom, there is no attempt in practice to reduce the great quantity of red fumes emitted (which is simply recovered and purified), but some steelmakers have tried to bring about stirring of the bath and slag by an additional means, consisting in putting into the solid bottom of the converter porous refractory plugs through which a neutral gas is blown from bottom to top, such as nitrogen or argon, for example, throughout most of the oxygen-blowing with the lance. This additional means gives advantageous results, such as good stirring of the bath, but presents certain disadvantages of its own:
First, such porous plugs can only blow neutral gases, since oxidizing gases and a fortiori pure oxygen would cause far too rapid wear of the porous refractory.
When selecting a neutral stirring gas, it is hard to make a choice between nitrogen, which is a cheap gas, but nitrides the bath and lessens the calorific power of the converter gases, and argon, which does not have the first of these disadvantages but is expensive.
Even with cheap nitrogen, the cost of the neutral gas consumed is significant.
With a neutral gas, the porous refractory plugs are worn quite slowly so that this is a usable blowing method in practice, but such wear is nevertheless quite fast so that such plugs do not have as long a life as the solid bottom and the refractory lining of the converter when blowing from above is used.
To sum up, this method using mixed blowing has advantages but also has considerable limitations.
The second type of pure oxygen blowing in a converter in a steelworks consists in using tuyeres, vertical or oblique, disposed in the refractory bottom of the converter, blowing from bottom to top, and constituted by at least two concentric tubes, the inner tube (or tubes) blowing an oxidizing gas, which can be pure oxygen, and the outer tube having running through it an agent for protecting the tuyere against wear in service.
This second type of pure oxygen blowing in a steelworks converter, from bottom to top, has several advantages over blowing with a lance, from top to bottom, among them and worthy of mention the following two:
A smaller amount of red, iron oxide fumes emitted during blowing.
Excellent stirring of the bath and slag during almost the whole operation, except for the last seconds, however, when the bath reaches low carbon contents. This has been remedied, though, by a very short final stirring by a neutral gas replacing the oxygen in the tuyeres, most often nitrogen, more rarely argon. This final, very short, stirring has none of the disadvantages noted hereinbefore for porous plugs.
However, complete blowing through the bottom has two disadvantages with respect to lance-blowing:
(a) the life of the bottoms provided with tuyeres for blowing, although in the process of being permanently improved, is still less than the life of the lateral refractory linings of the converter.
(b) Any advance in dephosphorization with respect to decarbonization is more difficult, even impossible, to obtain.
The idea then occurred to combine blowing by lance and blowing by tuyeres at the bottom in the same converter, so as to benefit from the advantages of both types of blowing, hopefully minimizing their disadvantages. In this instance, blowing oxygen through the tuyeres at the bottom under a pressure which increases as the carbon content of the bath decreases was even recommended.
But even if the disadvantages of blowing by lance were thus reduced, not to say removed, as far as the lack of stirring was concerned, for example, the disadvantage of the performance of the bottom with tuyeres being less good than that of lateral refractory lining remained. This disadvantage was particularly marked in the case of blowing with oxygen at increasing pressure in the tuyeres at the bottom, since the speed of wear of the tuyeres increases at low carbon contents in the bath with a given flow of oxygen, and even more so with an increased flow.